Examine source code of Simple dark square

{
  "name": "Cityscape",
  "manifest_version": 2,
  "version": "2.0",
  "theme": {
    "images": {
      "theme_frame": "8k firefox header.png"
    },
    "colors": {
      "toolbar": "rgba(245, 245, 245, 1)",
      "frame": "rgba(28, 28, 28, 1)",
      "tab_background_text": "rgba(240, 240, 240, 1)",
      "bookmark_text": "rgba(28, 28, 28, 1)"
    }
  }
}